Reliability of Wrist Heart Rate Monitors – BionicOldGuy

Devices that monitor our heart rates based on a measurement at the wrist are pretty prevalent nowadays, including Fitbit, Garmin watches, and Apple Watches. There are also devices that promise more sophisticated results such as heart rate variability from Whoop. These devices then tell us insightful things about our recent workouts or our activity throughout the day, how long we should recover before our next hard workout, and even how well we are sleeping.

The trouble is that the accuracy of the data all this information is based on is suspect. I first noticed this back when I had a Fitbit. It would often read higher than I suspected was correct, based on how hard I felt I was working. The same is now true of my Garmin watch. I can be just warming up, and it will tell me my heart rate is in the 130’s, which I know can’t be true because I’d really be huffing and puffing. I recently put it to a test. I happen to also have a Polar H10 heart rate strap. And a separate device (my Garmin bike computer). So on the same ride, I made sure my watch was not paired to the strap, and only had wrist data available for heart rate. My bike computer was paired to the strap so it was a chest-based measurement. In such a comparison, we expect the chest measurement to be more accurate, but I was blown away how much the two differed. The two charts from a three-hour ride are shown below. The max heart rate from the wrist measurement was 41 beats two high, and suspiciously showed up when I was just warming up. There are various other places in the charts where the wrist results are just not right.

I had been reading up on optimum training techniques based on heart rate, all of which sound quite scientific. But I now know that the data I was basing this on was untrustworthy (I usually just use wrist-based because it’s an extra step to put on the strap). In the future I will make sure I use the strap when I really want to check heart rate. I will also put more trust in how my workout feels (perceived level of exertion).